Conventionally, as a method for suppressing a false color that occurs in a high-frequency region of a subject, color interpolation has been performed on a target pixel by using surrounding pixels of the target pixel.
Referring to FIG. 16, a description will be given of an example of a process of performing color interpolation on image signals that have been output from pixels in a Bayer pattern in which red (R), green (G), and blue (B) filters are arranged.
In FIG. 16, the G filter positioned in the horizontal direction with respect to the R filter and in the vertical direction with respect to the B filter is called a G1 filter, and the G filter positioned in the vertical direction with respect to the R filter and in the horizontal direction with respect to the B filter is called a G2 filter. Also, the signals output from the pixels corresponding to the G1, G2, R, and B filters are represented by G1sig, G2sig, Rsig, and Bsig, respectively.
This image processing apparatus separates an image signal into an image signal constituted by G1sig, an image signal constituted by G2sig, an image signal constituted by Rsig, and an image signal constituted by Bsig, and performs an interpolation process so that all the pixels have a signal in image signals of the respective colors.
Then, the image processing apparatus obtains a correlation of values of G1sig or G2sig of pixels that are vertically adjacent to a target pixel of color interpolation and a correlation of values of G1sig or G2sig of pixels that are horizontally adjacent to the target pixel. If the correlation in the horizontal direction is higher than the correlation in the vertical direction, the image processing apparatus regards the value of Rsig−G1sig as a color-difference signal R−G, and regards the value of Bsig−G2sig as a color-difference signal B−G. In contrast, if the correlation in the vertical direction is higher than the correlation in the horizontal direction, the image processing apparatus regards the value of Rsig−G2sig as a color-difference signal R−G, and regards the value of Bsig−G1sig as a color-difference signal B−G as described in Japanese Patent Laid-Open No. 2002-300590.
Alternatively, the image processing apparatus regards an average value of the value of Rsig−G1sig and the value of Rsig−G2sig as a color-difference signal R−G by using a weighting coefficient corresponding to the difference between the value of Rsig−G1sig and the value of Rsig−G2sig. Likewise, the image processing apparatus regards an average value of the value of Bsig−G1sig and the value of Bsig−G2sig as a color-difference signal B−G by using a weighting coefficient corresponding to the difference between the value of Bsig−G1sig and the value of Bsig−G2sig as described in Japanese Patent Laid-Open No. 08-023541.
In this way, a process of reducing a false color has been performed by generating color-difference signals by adaptively selecting pixels to be used for color interpolation.
Also, when luminance signals are to be generated, image signals are generated by adaptively selecting pixels to be used for interpolation in accordance with a correlation of pixels positioned in the horizontal direction and the vertical direction with respect to a target pixel, whereby a process is performed while suppressing moire and preventing unnaturalness in an entire image as described in Japanese Patent Laid-Open No. 2007-336384.
However, even in the above-described process, correlations in the horizontal direction and the vertical direction are not correctly obtained in a region containing an image signal approximate to a Nyquist frequency or an image signal containing aliasing noise, so that a false color or moire may remain. Therefore, it is considered that there is room for improvement for further reducing a false color or moire.